Lots of reasons for this. If it's just not working, look at the cable and the hub/router/switches.

The most common and often overlooked reason for slow network and service response is naming problems. Many services look for names, timeout, and then go on without issuing an error message, sometimes not even in logs. If you have a slow dns lookup mechanism, it could be because of that and not actually a timeout error.

There are many different naming protocols including DNS, WINS, hosts, and lmhosts. All of these should be looked into if you have slow network responses or timeouts, especially if they occur at initial connection times, i.e. logon or transporting email.

If it's slow,good changes it's a hub/router/switch that got into a funk. Unplug the device and plug it back in. Don't push the reset button because that may erase the programming. Turning the thing off and on doesn't often work because on/off buttons these days don't really turn things off. Even unplugging may not reset them because of battery backups and non-volatile memories, but most decent devices have that way of resetting.